The field of the invention relates to control systems for maintaining engine air/fuel operation within the peak efficiency window of a catalytic converter.
Air/fuel control systems are known which are responsive to exhaust gas oxygen sensors positioned both upstream and downstream of a catalytic converter. Typically, a feedback variable is derived by integrating the output of a two-state exhaust gas oxygen sensor positioned upstream of the catalytic converter. This upstream exhaust gas oxygen sensor has a step change in its output, switching between rich and lean output states, at a preselected air/fuel ratio. Similarly, the downstream sensor is a two-state device with an output step change between rich and lean indicating steps occurring at a predetermined air/fuel ratio. An output from the downstream sensor biases the upstream feedback loop so that on average the engine's air/fuel ratio is aligned with the step change in the downstream sensor's output.
The inventors herein have recognized that the step output of the downstream sensor and corresponding predetermined air/fuel ratio may not be in alignment with the peak efficiency window of the catalytic converter.